This invention relates to radio frequency localization, in some embodiments using synthetic aperture techniques, and in particular relates to use of techniques in which emission from targeted objects are not necessarily synchronous with the receiver and/or emissions from a targeted object to the receiver follow multiple discrete paths, such as direct path and one or more reflected paths.
Synthetic Aperture Radar (SAR) is a technique in which, generally, successive pulses of radio waves are transmitted from a transceiver to “illuminate” a target scene, and the echo of each pulse is received and recorded at the transceiver. The antenna of the transceiver is generally mounted on a moving vehicle, such as a moving aircraft, so that the antenna's location relative to the target changes over time. Signal processing of the recorded radar echoes allows combination of recordings from the multiple antenna locations, thereby forming the synthetic antenna aperture and providing finer resolution image of the illuminated target scene than what would be possible with the given physical antenna at a single location. Note that in such systems, the signal transmitted from the antenna is synchronized with the received echoes by virtue of the transmitting and receiving sections of the system having a common time reference, for example, in the form of a common and relatively stable oscillator (i.e., stable in the time scale of the round trip time of the reflected signals).
Localization of radio frequency reflectors or transmitters (collectively “emitters”) may be complicated by the presence of reflecting structures that cause there to be multiple paths from a target object to a receiver. For example, in indoor localization, reflections from walls may introduce such multipath effects. As another example, in outdoor localization in urban environments, signals may be reflected from buildings causing multipath effects. In some situations, the reflected signals may have higher energy than the direct path from the target to the receiver.